The present invention relates to a method of mass spectrometry and a mass spectrometer.
In many applications very complex mixtures of compounds are analysed. Individual components within these mixtures are present with a wide range of relative concentrations and may be in the presence of large concentrations of matrix or endogenous background signals. This gives rise to a wide range of ion current intensities which are transmitted to the mass analyser and the ion detector. For many applications it is important to produce quantitative and qualitative data (in the form of exact mass measurement) for as many specific target analytes as possible. This puts very high demands on the dynamic range of the ion source, the mass analyser and the ion detection system employed in the mass spectrometer.
It is known that the addition of ion mobility separation to a mass spectrometer results in a concentration of the ion signal as ions from a particular analyte are delivered to the ion detector in a short period of time compared to the total ion mobility separation time. This ion concentration effect puts high demand on the ion detector and ADC recording system resulting in a reduced dynamic range.
A known method of controlling the intensity of a signal is to adjust the transmission or sensitivity of the mass spectrometer or the gain of an electron multiplier to keep the most intense species of ion within a specific mass to charge ratio range within the dynamic range of the ion detection system. This may be the base peak within a whole spectrum or a specific mass to charge ratio value in a targeted analysis. In this case it may not matter that signals from other mass to charge ratio values exceed the dynamic range of the detection system as long as they are separated from the target of interest.
U.S. Pat. No. 7,047,144 and U.S. Pat. No. 7,238,936 disclose methods of adjusting the gain of an ion detector based upon the intensity of the largest peak within a defined mass to charge ratio value. This known method of adjusting the gain is particularly prone to errors due to interference of background ions.
GB-2489110 (Micromass) discloses with reference to FIG. 2 an arrangement comprising an ion mobility separation device, an attenuation device and a Time of Flight mass analyser. Ions are subjected to a two dimensional separation and ions having a particular ion mobility and a particular mass to charge ratio are selectively attenuated.
US 2010/108879 (Micromass) discloses an arrangement comprising an ion mobility spectrometer and an ion gate. The operation of the ion mobility spectrometer and ion gate are synchronised so that only ions having a particular mass to charge ratio and a desired charge state are onwardly transmitted to a collision cell.
US 2006/020400 (Okamura) discloses a detector assembly having a current measuring device with a saturation threshold level.
GB-2502650 (Micromass) discloses selectively attenuating abundant or intense species of ions in a population of ions.
It is desired to provide an improved mass spectrometer and method of mass spectrometry.